What this lesson teaches
I can explain what drives wind direction and speed, including pressure differences, breezes and monsoons.
Syllabus K335, Topic 3.1, Key Question 4. Why do wind direction and wind speed vary across Earth's surface?
Make a guess
On a hot afternoon at the coast, which way does the breeze blow?
- From the sea towards the land.
- Straight down from the sky.
- There is no breeze in the afternoon.
- From the land out to sea.
Show the answer
From the sea towards the land.
By day the land heats faster. Air rises over the land, so pressure falls there and cooler sea air flows in.
Wind blows from high to low pressure; the bigger the pressure difference over a short distance, the stronger the wind.
Wind is air moving from high pressure to low pressure. The pressure gradient is how fast pressure changes over a distance. A steep gradient gives a strong wind. On a weather map, isobars join places with equal pressure. Close isobars mean strong wind.
By day, land heats faster than the sea. Air over the land rises, so pressure there is lower. Cooler air flows in from the sea. This is a sea breeze, and air returns out to sea higher up. At night, land cools faster. Air may flow from the land out to sea. This is a land breeze, and it is usually weaker. Strong prevailing winds can change or override both breezes.
Trees, buildings and rough ground slow the wind by friction. Valleys and gaps between buildings can funnel wind and speed it up. Over large areas, Earth's spin also bends the wind's direction. Winds are named by where they come from. A sea breeze blows from the sea.
- Wind
- Air moving from high to low pressure.
- Isobars
- Lines joining equal pressure; close lines mean strong wind.
- Sea breeze
- Daytime wind from the sea onto land.
Step by step
Land warms faster
By day, the Sun heats land faster than the sea.
Air rises over land
Pressure over the land becomes lower.
Sea air moves in
Air flows from the sea to the land: a sea breeze.
Worked example: Gradient needs distance
Suppose pressure falls 8 hPa over 100 km: that is 0.08 hPa per km. The same fall over 400 km is 0.02 hPa per km. With similar friction, the first, steeper gradient usually gives the stronger wind.
Watch out for this
A sea breeze blows out to sea.
Winds are named by where they come from. A sea breeze comes from the sea and blows inland.
Check your understanding
Two areas have the same pressure difference. Area A spans 50 km, and Area B spans 200 km. Which usually has stronger wind?
- Both, because the pressure difference is equal.
- B, because the air travels farther and speeds up.
- A, because its pressure gradient is steeper.
Show the answer
A, because its pressure gradient is steeper.
Yes. The same change over a shorter distance gives stronger wind.